Yes Roz, some cars do depreciate faster than others. There's lots of reasons, including the build quality of the car, and the car company's marketing strategy.
It doesn't matter much to me, as I tend to keep my cars until the wheels fall off, but for instance it affects lease companies big time, as they price their leases based on how much they think they can offload the car for in 3 years time. If you intend to change cars in a few years time, the resale value will have a big impact on your cost of ownership.
In 2005, it's predicted a current Porsche 911 Carrera will be worth 69% of its current price, ie. it will drop from $187,600 to $129,444. That's a loss of $19,285 per annum.
However that residual is better than you'll get for a Commodore or Falcon, which will drop to 50-59%. The more expensive models (Caprice, LTD) tend to lose more over the period.
In a quick scan, the best 3-year residual I found was the Mercedes-Benz CLK 320 Avant-Garde Cabriolet (74%). The worst residuals I found were the Ford Falcon LTD, the Honda NSX, and the Lamborghini Diablo which are all expected to lose more than half their value over the 3 year period.
Of course half the price of a Falcon is somewhat less than half the price of a Lambo..
What does that mean to me? If a 2-3 year old car has all the features I want, it's an attractive purchase because it's just been run in, the first owner fixed all the warranty problems, and I get anything up to a 50% discount! Perhaps a 3 year old Lambo...I'll note that in my doodad file.